This invention relates to an amplifying circuit with reduced idling current and expanded range of class-A operation.
In the prior art class-A amplifier, base bias is determined at such a value that operating current (collector current flowing when no signal is being applied) becomes one half of peak current (i.e., the middle point of the operating curve) so that amplifying elements are constantly kept in an active state both in positive and negative cycles of an input signal and linearity in amplification thereby is ensured. FIG. 1 shows a prior art class-A push-pull amplifier as an example of such class-A amplifier. In this amplifier, large base biases E1 and E2 are applied to transistors 10 and 12 of the push-pull construction.
Input-output characteristics of the class-A push-pull amplifier of FIG. 1 are shown in FIG. 2. The characteristics show that, owing to the application of the base biases E1 and E2 to the transistors 10 and 12, the transistors 10 and 12 are not cut off with resulting reduction in distortion.
The prior art class-A amplifier however is disadvantageous in that it requires a relatively large idling current and that the range in which it can perform the class-A operation is limited to a relatively narrow range and, if an input of a level exceeding this range is applied to the amplifier, one of the trasistors is cut off with resulting generation of distortion.
It is, therefore, an object of the invention to provide an amplifying circuit with reduced idling current and an expanded range of class-A operation.